1. Field of the Invention
The present invention relates to a packaging technique for the fabrication of polarized light emitting diodes.
2. Description of the Related Art
Light emitting diodes (LEDs) have been used in the last thirty years as indicator lamps, local illuminators, and optical transmitters, among their many applications. In the last ten years, high-brightness AlInGaN-based blue and green LEDs have been developed for, and have started to emerge in, general lighting and full-color display applications.
In terms of LED fabrication, because of the incoherent and unpolarized light emission from conventional LEDs, it is not essential to define a particular die orientation of an LED package when the die is attached to the package. In common LED fabrication, die orientation is only significant when an LED wafer is diced, that is why LED photolithographic patterning onto a wafer is carried out by aligning the patterns along crystallographic directions. This alignment process makes die separation reliable and results in higher production yield.
In the case of AlInGaN LEDs prepared on an insulating substrate (for example, sapphire), where two electrical contacts are made on one side of an LED die, die orientation in relation to the package is significant in terms of position of the positive and negative metal contacts. These alignments for reliable die separation and electrical contacts are common practice for any semiconductor devices, not necessarily only in LED fabrication. However, LED die alignment has never been considered in fabrication in terms of emitted light properties.
Internal electrical polarization is a unique property of the (Al,In,Ga)N system compared to other semiconductors used in optoelectronics, and this property originates in the hexagonal crystallographic structure of the (Al,In,Ga)N material system. FIG. 1 is a schematic of a generic hexagonal würtzite crystal structure 100 and typical crystallographic planes of interest 102, 104, 106, 108 with principal crystallographic axes or directions 110, 112, 114, 116 identified therein, wherein the fill patterns are intended to illustrate the planes of interest 102, 104 and 106, but do not represent the materials of the structure 100.
Electrical polarization is created in the hexagonal structure due to its lack of inversion symmetry along the c-axis. For example, in the case of GaN shown in FIG. 2, along the c-axis gallium atoms (cation, positively charged) and nitrogen atoms (anion, negatively charged) are positioned alternately and as a whole, electric neutrality is maintained. However, because of the lack of inversion symmetry there exists an internal electric field along the c-axis when the atoms are displaced from their ideal positions relative to each other along this axis. Since atoms in the AlInGaN system usually do not maintain their ideal positions, this polarization field almost always exists along the c-axis. For this reason, the c-plane is called a polar plane. Polarization fields do not exist along any of the a-axes or m-axes, due to the inversion symmetry along these particular axes. For this reason, a-planes and m-planes are called non-polar planes. For these planes, the polarization vector, which expresses direction and strength of polarization field, is parallel to the planes, since the net polarization vector is parallel to the c-axis.
AlInGaN materials are conventionally grown in the c-direction (direction along c-axis), and therefore on the c-plane. LEDs grown on the c-plane emit light with negligible light polarization. On this plane, the polarization field has no in-plane component and the isotropic mechanical stress within the c-plane in a quantum well (QW) structure of an LED does not change the nature of carrier recombination in the QW.
It has recently become possible to prepare AlInGaN LEDs on a-planes and m-planes. These LEDs exhibit linearly polarized light emission. The polarization field is in a particular direction (c-direction) in the plane, and the stress in the QW is anisotropic due to different degrees of lattice mismatch between the substrate and QW in the two perpendicular directions in the plane. The inventors have confirmed the emitted light from these non-polar LEDs is linearly polarized in a direction perpendicular to the c-axis.
Linearly polarized light is an electromagnetic wave that has its electric field only in one plane perpendicular to its propagation. Non-polarized light has its electric field evenly distributed in directions in planes perpendicular to its propagation. A principle application for polarized light is backlighting for liquid crystal displays (LCDs), in which LEDs are beneficial due to their compactness and energy efficiency compared to conventional cold cathode fluorescent tubes.
(Al,In,Ga)N LEDs prepared on a semi-polar plane have also been confirmed to emit polarized light. The projection of the polarization vector, which is parallel to the c-axis, lies in the semi-polar plane, similar to the non-polar plane case.
What is needed in the art are simplified methods of fabricating polarized LEDs and packaging such LEDs. The present invention satisfies those needs.